Electric double-layer capacitors (ELDCs) are large in electrostatic capacity, and excellent in charge/discharge cycle characteristics, and thus they are used as backup power sources in various equipments, including automobiles. For the ELDCs, use may be made of a polarizable electrode obtained by forming an active carbon with a binder resin, such as polytetrafluoroethylene, into a sheet form.
As a method of producing the active carbon used in the polarizable electrode, a method is proposed which method includes: mixing magnesium salt of an organic acid or the like, as a template, with an organic substance which is to be a carbon precursor, calcinating the resultant mixture, to prepare a composite of carbon and magnesium oxide (MgO), and removing the MgO template by elusion by treating the composite with an acid, thereby preparing a porous carbon (see Patent Literature 1). However, by this method, it is not possible to produce a nitrogen-containing porous carbon material in which nitrogen is made into a solid solution in the thus-prepared carbon skeleton.
Nitrogen-containing carbon materials are more suitable as electrode materials for EDLCs, and a nitrogen-containing carbon material is known which is prepared by using a polymer high in a nitrogen content, such as a melamine resin, as a raw material. Patent Literature 2 describes a method of producing a nitrogen-containing carbon using a swellable fluorine mica as a template, and a melamine resin and/or an acrylonitrile resin. Further, Patent Literature 3 proposes a method of producing a nitrogen-containing carbon by carbonization of a melamine resin foam.
In accordance with the demand for increasing the capacity of EDLCs, it is desired to increase the surface area of the nitrogen-containing porous carbon material, but a sufficient one has not been obtained by the above-mentioned methods. Further, since a melamine monomer that is a raw material of a melamine resin sublimates by a heat treatment, this is not directly used as a raw material for a nitrogen-containing porous carbon material. As a result, it is difficult to readily produce a nitrogen-containing porous carbon material.